The present invention relates to a method for separating a mixture of cation and anion exchange resins into their respective resin types.
In the purification of water and other liquids, to remove the undesirable ionic components therefrom, the liquid being purified is often contacted with both anion and cation exchange resins. Although the contaminated liquid can be sequentially contacted with one resin type and thereafter with the other resin type, the liquid is more conventionally contacted with a resin bed containing both the anion and cation exchange resins, i.e., a mixed resin bed.
In such mixed bed ion exchange operations, upon the exhaustion of the resins, e.g., the reduction in the capacity of the resins to a commercially impractical level, the cation and anion exchange resins are separated and the separated resins subsequently regenerated to restore ion exchange capabilities thereto. For the reason that a dilute caustic solution is employed as the regenerant for the exhausted anion exchange resin and a diliute acidic solution for the exhausted cation exchange resin, complete separation of the anion and cation resins is desired prior to regeneration. Any intermixing of the resins resulting from their incomplete separation causes the unseparated resins to be damaged and/or to remain in an exhausted condition following regeneration.
Conventionally, separation involves backwashing the mixed resin bed, i.e., fluidizing the mixed resin bed by passing a water wash up through the bed for a limited time, and thereafter allowing the resins to settle. Since the cation resin is generally more dense than the anion exchange resin, it settles first and stratification of the cation and anion exchange resins occur.
Unfortunately, when using such conventional separation techniques, the interface defining the boundary between the anion and cation exchange resins is often not distinct, i.e., a portion of the separated resin bed contains intermixed anion and cation resins. Therefore, following regeneration of the resulting separated resin bed using conventional regeneration techniques such as defined by U.S. Pat. No. 3,385,787; large, unseparated amounts of each resin remain in their exhausted state.
To facilitate more complete separation, the prior art methods disclosed in U.S. Pat. Nos. 3,429,807 and 3,634,229 propose using a sight glass or the like to monitor the degree of separation and the position of the interface marking the boundary between the layers of the cation and anion resin. Unfortunately, such techniques are not readily adapted to fully automatic facilities and are inherently inaccurate.
To effectively separate the resins in an automated facility, U.S. Pat. No. 4,120,786 proposes a method and apparatus for locating the interface of the anion and cation exchange resins by monitoring the pH of the resin bed, wherein a change in pH indicates the position of the interface. While such method can be employed to continuously monitor the position of the interface in an automated ion exchange facility, the interface remains relatively indistinct causing a portion of each resin to remain in its exhausted condition following regeneration.
In view of the stated deficiencies of the prior art methods, it remains highly desirable to provide a method for effecting complete separation of a resin bed comprising a mixture of anion and cation exchange resins and for accurately determining the position of each resin in the separated resin bed, thereby facilitating the regeneration of each resin type.